Voltage-controlled detectors such as ionization chambers, photomultipliers, semiconductor detectors including photodiodes of different types, silicon photomultipliers, etc., require a voltage difference applied between the grounded parts of the detector and the sensor output line; the electrical current or charge in such line is the measure of the signal. The condition of stable operation of the voltage-controlled detectors is such that the voltage applied to the sensor output line should remain constant and independent of the sensor's output current, which is equivalent to having a low differential input impedance (dV/dI) of the input circuit of the amplifier cascade connected to it.
A schematic of traditional first stage of the amplification cascade for voltage-controlled detectors is shown in FIG. 1. Shown in FIG. 1 is such a system 10 wherein 12 is the applied operational voltage for the detector 36, 34 is the sensor output line, 30 is the operational amplifier, providing signal output 32, with its power supply lines 26 and 28, and feedback chain impedances 22 and 24. The traditional schematics include a power circuit (load) resistor 14, a decoupling input capacitor 16, input capacitance 18, and an input resistance 20 for the amplifier circuit. These elements present sources of extra noise and extra frequency-dependent signal distortions that ultimately affect the accuracy of the readings. There thus remains a need for a low noise readout circuit for voltage-controlled detectors.